1. Field of the Invention
The invention relates generally to the field of ultrasonic metal welding, which includes ultrasonic wire bonding, and relates more particularly to an apparatus and method for monitoring and analyzing the quality of the ultrasonic wire bonding process.
2. Description of the Relevant Art
Ultrasonic wire bonding is used extensively throughout the semiconductor industry to form electrical interconnections to integrated and hybrid circuits. In the process of ultrasonic wire bonding, a metallurgical bond is formed through the proper transmittal of ultrasonic vibrations to the bond interface between a wire and a bonding pad. In general, the bonding process involves the following steps: (1) a wire is placed in contact with a bonding pad, and a tool is placed on top of the wire; (2) the tool applies pressure against the wire, forcing the wire against the bonding pad; (3) an ultrasonic transducer is energized by a high frequency power signal to apply an ultrasonic vibration to the tool, which causes the tool to oscillate in the plane of the bonding pad; (4) the friction between the wire and the bonding pad is overcome, and the wire scrubs against the surface of the bonding pad, causing localized softening and plastic flow of the wire and/or bonding pad; (5) after several milliseconds of scrubbing, the transducer is deenergized, which stops the motion of the tool and wire and allows a weld to form between the wire and the bonding pad; and (6) the tool is lifted away from the now bonded wire. Key parameters for the ultrasonic bonding process include the frequency, duration, and amplitude of the ultrasonic vibrations, the pressure exerted by the tool, and the friction between the wire and the bonding pad, which is influenced by the materials of the wire and bonding pad, by their state of cleanliness, and by the ambient environmental conditions. The precise shape and surface finish of the bonding tool also plays a major role.
Certain factors may cause the bond to be defective. A bond may not form properly if, for example, the bonding pad or the wire is not clean. If the transducer is energized for too long a time, the bond may not form properly due to overheating of the materials or fracturing of the substrate. If the wire feed mechanism is faulty, or if the tool fails to contact the bonding pad, no bond will be formed.
Even if good bonds are produced at the beginning of a production run, there is no guarantee that good bonds will continue to be produced throughout the run. Tool wear, material build-up on the tool, tool replacement, variations in ambient temperature and humidity, and variations in the wire and bonding pad materials are factors that may contribute to poor bonding even though good bonds were produced initially. As a result of the foregoing, proper quality control procedures dictate that the quality of the bonding operations be constantly monitored. Continual quality monitoring is especially important in the case of automatic bonders, where numerous bonds are made in rapid succession, because the failure to detect faulty bonds may ruin a large number of circuit devices.
Prior ultrasonic bond quality monitors have utilized a variety of techniques in their attempts to reliably monitor the quality of the ultrasonic bonding operation. One such prior art apparatus, disclosed in U.S. Pat. No. 3,302,277 issued on Feb. 7, 1967 to Pruden, et al., measures bond quality by monitoring the voltage drop of a transistor contained on the substrate to which the wire is to be bonded. A bond quality monitor that measures bond quality by monitoring the amplitudes of the transverse and tangential motions of the bonding tool is disclosed in U.S. Pat. No. 3,890,831 issued on June 24, 1975 to Cusick, et al. Another bond quality monitor, disclosed in U.S. Pat. No. 4,040,885 issued on Aug. 9, 1977 to Hight, et al., measures bond quality by detecting the amplitude of the bonding tool during the bonding operation and comparing it to the amplitude of the bonding tool under no-load conditions. Certain bond quality monitors base their technique on the measurement of the electrical impedance of the ultrasonic transducer, including the apparatus disclosed in U.S. Pat. No. 4,341,574 issued on July 27, 1982 to Landes. The Landes apparatus determines when to de-activate the ultrasonic transducer at the conclusion of the bonding operation by detecting the zero-crossing of the second derivative of the impedance. Also, U.S. Pat. Nos. 3,636,456 issued on Jan. 18, 1972 and 3,852,999 issued on Dec. 10, 1974 to Wright, and 3,693,158 issued on Sept. 19, 1972 to Uthe, all of which are assigned to the assignee of the present invention, all disclose bond quality monitors that monitor the impedance of the ultrasonic transducer.
Prior art bond quality monitors, such as those disclosed in the above listed references, fail to reliably monitor the quality of bonding operations in the real-world environment of the production floor. These bond quality monitors attempt to create an absolute measurement of bond quality, which is an approach that is doomed to failure due to unavoidable variations in process parameters and environmental conditions.